In an optical communication system, an optical transmitter error correction code encodes a SONET/SDH flame of OC-48/STM-16 or OC-192/STM-64 and sends it onto an optical fiber transmission line, and an optical receiver corrects transmission errors using an error correction code and outputs the SONET/SDH flame of OC-48/STM-16 or OC-192/STM-64. As an error correcting system, a foward error correction (FEC) system has been known. An FEC system having more advanced error-correcting ability is also well known (see, for instance, T. Mizouchi, et al., “Transparent multiplexer featuring super FEC for optical transport networking”, proceedings of SubOptic 2001, pp. 484–487).
Generally, quality of an optical transmission line is unstable, and the unstableness appears more clearly as the transmission line distance becomes longer and the bit rate becomes higher. By introducing error-correcting technology with high correcting ability, it is possible to obtain transmission of practically error-free data even in a transmission line having poor quality. For instance, in an optical transmission line which quality varies in a range from 2×10−3 to 4×10−3 with its average quality of 3×10−3, an error rate after error correction varies between an error-free state and a frame-loss state.
Also, an optical transmission system, specifically an optical transmission system of trunk lines is designed to be able to select from a plurality of lines or routes to prepare for line fault. An electric crossconnector or a photonic crossconnector is well known as a means to switch traffic.
A large number of transmission errors can be corrected by the FEC system. Since high correction ability is obtained in the error correcting technology described in the above paper, it is possible not only to realize practically error-free transmission in a wide range but also to use optical transmission lines having inferior quality.
In this error correcting technology, however, an error rate after error correction rapidly increases if an error rate before error correction exceeds a certain threshold value (see FIG. 6 of the above-described paper). For example, when an error rate before error correction deteriorates from 4×10−3 to 8×10−3, an error rate after error correction greatly worsens from 10−11 to 10−2.
In an optical transmission line requiring a high error correcting capability, a minor deterioration of transmission characteristics is likely to cause disabled conditions. Accordingly, when a transmission error sufficiently serious to cause such disablement occurs in a line, the line is switched to another line by a crossconnector.
However, any means to easily monitor such deterioration in transmission characteristics has not been proposed yet. Conventionally, an abnormal condition in a transmission line is merely detected on a result obtained by restoring a data carried by an optical carrier. Therefore, in a conventional system, alarm signals are output from many parts to inform deterioration of transmission characteristics in an optical fiber transmission line.
Conventionally, a crossconnector switches lines manually or automatically when an abnormal condition occurs in a transmission line as mentioned above. Some electric crossconnectors have an error monitoring function. When such an electric crossconnector is employed, it is possible to make the electric crossconnector automatically switch working lines when the error rate reaches a certain value. When such an automatic switching system is used, however, transmission characteristics often become unstable because the line switching causes chattering.
In addition, since signals are transmitted in an inferior transmission quality until the line having a high error rate is switched to another line, packet retransmission is repeated. This increases loads in the network.
In a photonic crossconnector, a special electric device must be installed to calculate an error rate of input optical signals. This reduces merits to utilize a photonic crossconnector instead of an electric crossconnector.